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Basic primer on spontaneous parametric down-conversion: it's an optical phenomenon where a photon of a given frequency (energy) is split into two entangled phot

Basic primer on spontaneous parametric down-conversion: it's an optical phenomenon where a photon of a given frequency (energy) is split into two entangled photons whose frequencies add up to the original photon. The resulting frequencies differ so you can separate the photons spatially by refracting them. Has long been a workhorse for experiments requiring a pair of entangled particles (Bell tests and such). You can buy SPDC crystals online for about $500 these days[0]. I guess they managed to SPDC a photon into three photons instead of two. [0] https://www.newlightphotonics.com/SPDC-Components/405nm-Pumped-SPDC-Components https://www.newlightphotonics.com/SPDC-Components/405nm-Pump...

anfractuosity 7y ago
Wow, that seems really interesting that you might not need a very expensive setup to demonstrate entanglement then. I'm just reading more about the bell test. Edit: I just found this talk https://www.youtube.com/watch?v=tn1sEaw1K2k https://www.youtube.com/watch?v=tn1sEaw1K2k which looks really interesting 'Shanni Prutchi Construction of an Entangled Photon Source'. It looks like she's using SPADs for detection of the photons.
- ahelwer 7y ago
  Yeah, the expensive/hard part is generating & measuring only a single photon at a time. Of course if you fire a laser through an SPDC crystal a bunch of photon pairs will come out entangled, but that isn't terribly useful for most entanglement experiments.
  - anfractuosity 7y ago
    Ah Gotcha, I'm intrigued wiki says "SPDC allows for the creation of optical fields containing (to a good approximation) a single photon. As of 2005, this is the predominant mechanism for an experimenter to create single photons", do you know of a simple explanation on how single photons are generated using the crystal + laser?
    - fsh 7y ago
      Most quantum optics experiments are done by simply attenuating laser pulses until they have on average much less than one photon per pulse. This way the probability of having more than one photon is very low (at the cost of reduced count rates since most pulses don't contain any photons). These pulses can then be used to create entangled photon pairs using SPDC. There are also several ways of creating pulses that contain exactly one photon, but this is way more complicated.
- lisper 7y ago
  It's not enough to produce entangled photons, you also have to be able to detect them individually. That's the expensive part.
deleted 7y ago
[deleted]
ianai 7y ago
Could you get a similar effect shooting electrons at something? Like is there a known path to do that.
daxfohl 7y ago
If they get back together, then do they join to create the old photon? What if you stick one in a stronger gravitational field for a while and then later you stick them back together but maybe the phases of something are out of sync due to time dilation (or maybe they're in sync but just one has experience an extra second of life)? Does the resulting photon become entangled with the second-ago rendition of itself? Are we able to control an entangled photon well enough to run a process like this? Sorry for the interested bystander questions.
- turndown 7y ago
  Wouldn't interaction with the particles that modified that portion of spacetime destroy the entanglement?
- californical 7y ago
  Photons don't experience time dilation as they don't even experience time (since they're moving at the speed of light).
  - daxfohl 7y ago
    Ack, can't believe I missed that! Still, the same question could be asked for other particles that can be split in the same manner ... if such particles exist.
  - evanb 7y ago
    They do blue or red shift in response to gravitational gradients and the like.
    - Faaak 7y ago
      Yes, but from your point of view, not theirs
  - triplesex_ 7y ago
    Why do we associate the speed of time with the speed of light? Is this even valid assumption?
sriku 7y ago
What's the underlying physical mechanism? The photon is absorbed and reemitted as two?
- anonytrary 7y ago
  I guess one could draw the Feynman diagram for this interaction, but I don't think it makes a whole lot of sense to talk about a physical "mechanism". If the quantum numbers are conserved (energy, momentum, angular momentum, etc.) then it can just happen.
  - jrwr 7y ago
    Confirmed, Its magic!
- ganzuul 7y ago
  Can't say about the physics, but with my recent approximate knowledge of algebraic structures I think it has to mean that QM is an OK division algebra, meaning something is invertible. If the invertible thing is a matrix then it might have nice computational implications.

anfractuosity 7y ago

Wow, that seems really interesting that you might not need a very expensive setup to demonstrate entanglement then. I'm just reading more about the bell test. Edit: I just found this talk https://www.youtube.com/watch?v=tn1sEaw1K2k https://www.youtube.com/watch?v=tn1sEaw1K2k which looks really interesting 'Shanni Prutchi Construction of an Entangled Photon Source'. It looks like she's using SPADs for detection of the photons.

ahelwer 7y ago
Yeah, the expensive/hard part is generating & measuring only a single photon at a time. Of course if you fire a laser through an SPDC crystal a bunch of photon pairs will come out entangled, but that isn't terribly useful for most entanglement experiments.
- anfractuosity 7y ago
  Ah Gotcha, I'm intrigued wiki says "SPDC allows for the creation of optical fields containing (to a good approximation) a single photon. As of 2005, this is the predominant mechanism for an experimenter to create single photons", do you know of a simple explanation on how single photons are generated using the crystal + laser?
  - fsh 7y ago
    Most quantum optics experiments are done by simply attenuating laser pulses until they have on average much less than one photon per pulse. This way the probability of having more than one photon is very low (at the cost of reduced count rates since most pulses don't contain any photons). These pulses can then be used to create entangled photon pairs using SPDC. There are also several ways of creating pulses that contain exactly one photon, but this is way more complicated.
lisper 7y ago
It's not enough to produce entangled photons, you also have to be able to detect them individually. That's the expensive part.

deleted 7y ago

[deleted]

ianai 7y ago

Could you get a similar effect shooting electrons at something? Like is there a known path to do that.

daxfohl 7y ago

If they get back together, then do they join to create the old photon? What if you stick one in a stronger gravitational field for a while and then later you stick them back together but maybe the phases of something are out of sync due to time dilation (or maybe they're in sync but just one has experience an extra second of life)? Does the resulting photon become entangled with the second-ago rendition of itself? Are we able to control an entangled photon well enough to run a process like this? Sorry for the interested bystander questions.

turndown 7y ago
Wouldn't interaction with the particles that modified that portion of spacetime destroy the entanglement?
californical 7y ago
Photons don't experience time dilation as they don't even experience time (since they're moving at the speed of light).
- daxfohl 7y ago
  Ack, can't believe I missed that! Still, the same question could be asked for other particles that can be split in the same manner ... if such particles exist.
- evanb 7y ago
  They do blue or red shift in response to gravitational gradients and the like.
  - Faaak 7y ago
    Yes, but from your point of view, not theirs
- triplesex_ 7y ago
  Why do we associate the speed of time with the speed of light? Is this even valid assumption?

sriku 7y ago

What's the underlying physical mechanism? The photon is absorbed and reemitted as two?

anonytrary 7y ago
I guess one could draw the Feynman diagram for this interaction, but I don't think it makes a whole lot of sense to talk about a physical "mechanism". If the quantum numbers are conserved (energy, momentum, angular momentum, etc.) then it can just happen.
- jrwr 7y ago
  Confirmed, Its magic!
ganzuul 7y ago
Can't say about the physics, but with my recent approximate knowledge of algebraic structures I think it has to mean that QM is an OK division algebra, meaning something is invertible. If the invertible thing is a matrix then it might have nice computational implications.